There are at least two known techniques for achieving the transmission of multiple data streams in a single medium. The first is "time domain multiplexing", or "TDM", in which each of the multiple individual data streams to be combined is sampled and assigned a specific timed space in a new data stream. This new data stream must also include some form of synchronization information that serves to identify the beginning of the data packages. TDM is illustrated in FIG. 1.
Typically, the data sampling rate is eight to ten times that of the highest speed data stream to be sampled to reduce the jitter or time errors incurred during the sampling process. The addition of overhead for the synchronization information runs the bandwidth of the new data stream to a much higher rate than any of the individual data streams.
The second technique is a process in which each individual data stream to be combined is used to modulate an individual carrier. The modulated carriers are then transmitted in a manner similar to that used by individual radio stations in making transmissions. As illustrated in FIG. 2, this technique requires that the total bandwidth be sufficiently wide to accommodate the sum of the carriers and their respective data-bearing sidebands.
It is apparent that the primary disadvantage of both of the techniques described above is the large bandwidth needed to transmit the signals. With the first technique, the bandwidth requirement is due to the data sampling process itself and the overhead required for synchronization in the demultiplexing process. With the second technique, the bandwidth requirement is due to the spectrum spread needed for the individual carriers and their respective sidebands.
Therefore, what is needed is a technique for combining multiple asynchronous data signals in a minimum of bandwidth.